Abstract
This paper describes a search for beyond the Standard Model decays of the Higgs boson into a pair of new spin-0 particles subsequently decaying into π-quark pairs, π»βπβ’πβ(πβ’
Β―
π
)β’(πβ’
Β―
π
), using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider at center-of-mass energy βπ =13ββTeV. This search focuses on the range 15ββGeVβ€ππβ€30ββGeV, where the decay products are collimated; it is complementary to a previous search in the same final state targeting the range 20ββGeVβ€ππβ€60ββGeV, where the decay products are well separated. A novel strategy for the identification of the π βπβ’
Β―
π
decays is deployed to enhance the efficiency for topologies with small separation angles. The search is performed with 36ββfbβ1 of integrated luminosity collected in 2015 and 2016 and sets upper limits on the production cross section of π» βπβ’π β(πβ’Β―π)β’(πβ’Β―π), where the Higgs boson is produced in association with a π boson.